Command substitution with string substitution

You can use pipe | with sed, example:

$ echo ABC | sed 's/B/_/'
A_C

Or variable substitution, all explained here, and especially at "Variable expansion / Substring replacement" section, and below an example from me:

$ var=ABC
$ echo ${var//B/_}
A_C

Syntax ${...} only allows referencing a variable (or positional parameter), optionally combined with parameter expansion.

Syntax $(...) (or, less preferably, its old-style equivalent, `...`), performs command substitution, which allows embedding arbitrary commands to whose stdout output the expression expands.

Thus, you could combine the two features as follows:

echo "$(lsOutput=$(ls); echo "${lsOutput//foo/bar}")"

^{Note the uncomplicated nested use of $(...), which is one of the main advantages over `...`, whose use would require escaping here.}

That said, any variables you define inside the command substitution are confined to the subshell that the command runs in anyway, so you could make do with just a command that produces the desired output, given that it is only the stdout output that matters.

echo "$(ls | sed 's/foo/bar/')"